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BIG PHYSICS, BIG QUESTIONS –

Curiosity rover finds ancient lake in Martian crater

By Lisa Grossman

As it might have looked (Image&colon; NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS)

Gale Crater on Mars was once a large lake that could have stuck around long enough for life to get started. New observations from the Curiosity rover, which has been driving around the now-dry crater floor since August 2012, show evidence of multiple cycles of water flowing into a large, shallow lake that could have lasted tens of millions of years.

Since shortly after it landed, Curiosity has been driving toward Mount Sharp, the 5-kilometre-high mountain at the crater’s centre. It reached the mountain’s foothills in September, and the team is busy drilling and analysing the rocks there in pursuit of the rover’s primary mission&colon; finding signs that Mars was once hospitable to life.

The view from the road looked optimistic. On the way to Mount Sharp, Curiosity found evidence for flowing rivers and fresh water where simple microbes could have made themselves at home.

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But two major questions remained&colon; was that water there long enough to support the emergence of life? And how did Mount Sharp form in the first place?

Ancient basin

In a news conference today, the rover team presented a possible answer to both questions&colon; Gale Crater was filled up with a lake that dried out and reappeared several times in the distant past, laying down the sediments that make up Mount Sharp in bursts of hundreds of thousands to millions of years.

In its drive towards the mountain, Curiosity first encountered conglomerate rocks full of pebbles that were probably deposited by rivers. But as it continued south and uphill, the landscape changed to sandstones that were all tilted in the same direction&colon; towards the mountain.

“This presents a certain paradox,” says project scientist John Grotzinger of the California Institute of Technology in Pasadena. “If Mount Sharp had been there and water had been flowing off Mount Sharp, it would be flowing downhill to the north. But the rocks that are exposed show the water flowing to the south.”

The resolution of the paradox is that Mount Sharp was not there when the water started flowing. Instead, water flowed from the crater rim toward the interior, filling up an ancient lake.

“On Earth, one of the key places where we find these inclined beds are at the mouths of river channels where they feed into lakes,” said Curiosity team member Sanjeev Gupta of Imperial College London. The landscape looks like small deltas stacked on top of each other, suggesting they formed over several cycles of varying climate.

Mountain of sediment

At the base of Mount Sharp, sandstone layers become much thinner and flatter, as if they were laid down more slowly and without strong currents swirling them around.

NASA’s Mars rover Curiosity catches a glimpse of Whale Rock, which shows structures that indicate water once flowed over a loose bed of sediment. (Image&colon; NASA/JPL-Caltech/MSSS)

“We think that’s what we’re seeing here&colon; lake floor deposits,” Grotzinger says. Cycles of wet and dry periods could have filled up the crater with sediment that was later eroded, leaving the mountain that remains today.

Sedimentary layers of rock on the lakebed (Image&colon; NASA/JPL-Caltech/MSSS)

This is something that the team suspected even before landing – “that’s why the landing site was selected,” Grotzinger says. But seeing it from the ground makes them much more confident. “There was no way to have recognized this from orbit,” he says.

But the ancient lake raises a new mystery&colon; how did Mars, which now locks all of its water up in polar ice caps, have enough water in its atmosphere to keep the lake in Gale Crater from evaporating for long periods of time?

“The climate system must have been loaded with water,” says deputy project scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Pasadena. If Gale Crater was liquid for millions of years, Mars may have needed oceans and a hydrological cycle. If the lake came and went in shorter periods, that could be an indication of climate cycles caused by volcanic activity or crater impacts. “The question is, could temporary climate fluctuations form what we see geologically, or do we need longer term warm wet climate?”